GitLab

- Plumbed from EmbeddedWorkerDevToolsAgentHost to
 ServiceWorkerVersion to flip 'DevToolsAttached' flag
- Added EmbeddedWorkerInstance::StopIfIdle() which doesn't stop
 the worker if devtools is attached
- Changed stop-worker-timer code to call StopIfIdle() instead of Stop()
- Added a hook in EmbeddedDevToolsManager that is called when we
 didn't stop the worker (to show an educational console message)

BUG=429582
TEST=EmbeddedWorkerInstanceTest.StopWhenDevToolsAttached

Review URL: https://codereview.chromium.org/761923004

Cr-Commit-Position: refs/heads/master@{#308332}